Light olefins such as ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide. The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenate-to-olefin (OTO) or specifically to methanol-to-olefins (MTO) process, where methanol is converted to primarily ethylene and/or propylene in the presence of a molecular sieve catalyst.
Various byproducts are produced in the MTO reaction process. These byproducts may include components that are heavier than propane and propylene, such as C4+ components (olefinic and aliphatic) as well as multiply unsaturated components such as acetylene, methyl acetylene and propadiene. Oxygenate compounds such as alcohols, aldehydes, ketones, esters, acids and ethers in the C1 to C6 range as well as trace quantities of aromatic compounds may also be formed in MTO reactors or in MTO effluent processing. Additionally, a small amount of oxygenate from the feedstock, e.g., methanol and/or dimethyl ether (“DME”), can pass through the MTO reactor with the product effluent without being converted to desired product. As a result of oxygenate synthesis and/or oxygenate “pass through” in an MTO reactor system, the effluent from an MTO reactor can contain undesirably high concentrations of oxygenate compounds. These oxygenates, particularly light oxygenates, are in amounts that would make the ethylene and propylene off-specification for their preferred dispositions, e.g., polymerization.
Various processing schemes have been developed for separating one or more of these components from non-MTO effluent streams. For example, U.S. Pat. No. 5,336,841 to Adams is directed to a process for removing oxygenates from a C4 raffinate stream from an MTBE plant. A back-cracking catalyst is placed into the bottom of an oxygenate removal column, which converts any MTBE or tertiary butyl alcohol contained therein back to their original components of isobutene and methanol or water. The raffinate stream is first subjected to a water wash to remove the gross amounts of methanol and DME.
U.S. Pat. No. 5,122,236 to Smith et al. is directed to a process for removing DME and methanol impurities from a C4 hydrocarbon stream without substantial loss of C4 hydrocarbons by fractionating a C4 hydrocarbon stream containing DME and methanol at low levels, e.g., less than 5 weight percent, to produce an overhead of about 20 to 40 volume percent of the C4 stream, condensing the overhead, contacting the condensed overhead with about 1 to 5 volumes of water, thereby removing a portion of the DME and methanol from the C4 stream, returning substantially all of the C4 stream, except the small amount solubilized in the water, to the fractionation and flashing the solubilized DME and hydrocarbons from the water.
U.S. patent application Ser. No. 10/292,232 filed Nov. 12, 2002, the entirety of which is incorporated herein by reference, is directed to a particularly desirable process for recovering C4 olefins from a product stream comprising C4 olefins, dimethyl ether and C5+ hydrocarbons. The process includes first separating out C5+ hydrocarbons and coboiling oxygenates, if any, from a stream comprising C5+ hydrocarbons, DME and C4 hydrocarbons. By first separating out the C5+ hydrocarbons and coboiling oxygenates, a more efficient separation of DME from C4 olefins by water wash is obtainable.
Although a variety of processes have been described for separating C4+ components from C3− components, separation schemes for efficiently recovering ethylene and propylene from other C3− components in a mixed effluent stream have not been widely described and have heretofore proven generally inefficient. Specifically, recovery of ethylene and propylene from lighter less desirable components, particularly from DME, has proven inefficient when the effluent stream contains a mixture of methane, DME, ethane, ethylene, propane and propylene. Thus, a need exists for efficiently separating ethylene and propylene from an MTO reaction system effluent stream containing these C3− components, or from a similar effluent stream derived from another reaction process.